Embodiments of the invention relate generally to magnetic resonance (MR) imaging, and more specifically, to an RF shield configured to prevent the generation of high temperature profiles on the surface thereof resulting from eddy current heating, so as to minimize impact on the performance of thermally sensitive parts, such as a positron emission tomography (PET) detector array in a hybrid PET-MRI system.
MR imaging involves the use of magnetic fields and excitation pulses to detect the free induction decay of nuclei having net spins. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a RF magnetic field (excitation field B1) which is in the x-y plane, i.e. perpendicular to the DC magnetic field (B0) direction, and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
PET imaging involves the creation of tomographic images of positron emitting radionuclides in a subject of interest. A radionuclide-labeled agent is administered to a subject positioned within a detector ring. As the radionuclides decay, positively charged particles known as “positrons” are emitted therefrom. As these positrons travel through the tissues of the subject, they lose kinetic energy and ultimately collide with an electron, resulting in mutual annihilation. The positron annihilation results in a pair of oppositely-directed gamma rays being emitted at approximately 511 keV.
It is these gamma rays that are detected by the scintillators of the detector ring. When struck by a gamma ray, each scintillator illuminates, activating a photovoltaic component, such as a photodiode. The signals from the photovoltaics are processed as incidences of gamma rays. When two gamma rays strike oppositely positioned scintillators at approximately the same time, a coincidence is registered. Data sorting units process the coincidences to determine which are true coincidence events and sort out data representing deadtimes and single gamma ray detections. The coincidence events are binned and integrated to form frames of PET data which may be reconstructed into images depicting the distribution of the radionuclide-labeled agent and/or metabolites thereof in the subject.
In combination PET-MRI systems, the RF shield associated with the MRI scanner is positioned in between the RF body coil and the gradient coil to help prevent the high amplitude RF field being radiated out, with the PET detector array being placed outside the RF shield in order to shield the sensitive detector array from the RF field. Depending on the proximity of the RF shield to the gradient coil and the type of gradient pulsing sequence applied, large amount of eddy-currents are created on the RF shield surface, with the pattern of these eddy current more or less mirroring the primary gradient current pattern. The eddy current generated on the RF shield produces heat that create high temperature profiles that affect the performance of any thermally sensitive parts located on or near the RF shield, such as the PET detector modules. The eddy current generated on the RF shield also raises the overall temperature of the patient bore, which may potentially cause discomfort to a subject being imaged.
It would therefore be desirable to provide an RF shield that prevents the generation of high temperature profiles on the surface of the RF shield resulting from eddy current heating, such as by disrupting larger eddy current profiles and any azimuthal generation of eddy current and by preventing the build-up of axial currents on the shield. It would also be desirable for the RF shield to still provide the necessary amount of shielding to the PET detector array and maintain the RF coil performance and image quality.